1. Technical Field
Aspects of this document relate generally to an apparatus and method for zoom alignment of clip-on electro-optic sights.
2. Background Art
Current clip-on sights are designed and manufactured as an un-zoomed (1.0×) or zoomed-out optical magnification sight. The un-zoomed optical magnification sight provides the necessary light amplification or thermal imagery and may be placed in front of a bore-sighted day optic without introducing any bore-sight shift to the combined optical system. Current clip-on sight must have the following two optical properties well established. First, the optical magnification must be very close to 1.0×. Usually the design nominal value is 1.0 with an allowable tolerance of 3%, so that magnification values ranging from a low of 0.97 to a high value of 1.03 will result in systems with acceptable performance. And second, the input optical axis and the output optical axis of the sight must be aligned. The angular tolerance for this determines the ultimate accuracy of the clip-on sight. Usually an alignment tolerance of within 1.0 minutes of arc is required to provide the accuracy for aimed weapons firing.
When these two values are achieved at the factory, a fielded clip-on sight provides a very simple and reliable device for the user. The clip-on sight is installed in front of the day optic and switched ON. The user looks through the day optic and sees an enhanced (intensified or thermal as the case may be) image of the scene, with the same aiming reticle superimposed that he is used to using with the day optic in normal day time operations.
FIGS. 1A and 1B illustrate a comparison of an object as viewed through only a day sight (FIG. 1A) and through a combined day sight and un-zoomed (1.0×) clip-on sight (FIG. 1B). As shown, a properly designed, manufactured 1.0× clip-on sight provides the essential low light imagery with no change of boresight alignment.
Small mechanical misalignments between the optical axis of the day sight and the clip-on sight will always exist. This misalignment is due to any combination of the following factors: day sight optical axis to weapon bore; weapon bore to mounting rail; and/or rail (weapon mount) mechanical axis clip-on sight optical axis. These factors can result in a small misalignment between the day sight and the clip on sight that do not cause an image shift, because of the inherent accuracy (magnification and alignment) built in to the clip-on sight, but result in the day sight looking at slightly different parts of the display. That is, the center pixel of the display is not necessarily going to be coincident with the day sight cross-hair. This will cause an aiming error when the zoom operation is performed.
Thus, in practice this means that the day sight reticle cross-hair is not pointing directly at the center pixel of the electronic display. When the zoom operation is performed, the displayed image is magnified, with only the image displayed on the center pixel remaining invariant. The further the day sight reticle is from the center display pixel, the greater the error in image displacement when the image is zoomed. Therefore, as depicted in FIGS. 2A and 2B, mechanical misalignment can cause boresight shift when the sight is used in the 2.0× zoomed mode. As shown, in the un-zoomed (1.0×) mode shown in FIG. 2A, the cross hairs of the reticle are over the bottom left of the truck windshield. In the 2.0× zoomed mode shown in FIG. 2B, the cross-hairs of the reticle are over the center of the passenger door of the truck, rather than the bottom left of the truck windshield. Such misalignment is problematic to user of the weapon.
Accordingly, existing sights rely on factory alignment tolerances to determine the overall accuracy of the system. Even more, some systems warn the user that the zoomed image will result in aiming errors, that it is for target observation tasks only, and/or that the aiming/firing should be performed in the un-zoomed (1.0×) mode only.